1. Field of the Invention
The present invention relates to an electro-photographic apparatus, and more particularly, to a method and apparatus for development using a dry two-component developer.
2. Description of the Related Art
Conventionally, a magnetic brush development method using a two-component developer consisting of magnetic carriers and toners has been employed in an electro-photographic apparatus. A development apparatus using the method normally has a magnet roller having a magnetic body including a plurality of magnetic poles and a development sleeve which is a rotatably supported cylindrical developer supporter. The development apparatus develops by holding magnetic carriers having toners on a surface of the development sleeve and carrying them to a development area. On the other hand, a one-component development method in which development is performed by using only magnetic toners or non-magnetic toners without magnetic carriers has also been employed. The one-component development method is similar to the two-component development method in respect to development by holding toners on a surface of a development sleeve and carrying them to a development area, but different from it with respect to detailed structures and a means of charging toners, etc.
In such a development apparatus, it has been proposed to improve image quality by improving of toner carrying performance by increasing surface roughness of a development sleeve using a one-component development method as described in Japanese Examined Application Publication No. 64-12386. A method to improve performance of carrying toners by increasing surface roughness of a development sleeve using a two-component development method has also been proposed as described in Japanese Laid-Open Patent Application No. 5-19632.
However, the methods described above presuppose non-contact development and that the quantity of developer on a developer supporter is controlled to be constant by using a developer quantity controller in the form of a bar. A non-contact development method using a developer quantity controller which is made from materials having rigidity or rigidity and magnetic properties have problems in providing enough developer on a developer supporter. Especially, size of carrier particles needs to be smaller to meet recent requirement for high image quality and downsizing. However, when size of carrier particles is made smaller, fluidity of the particles tends to be lower, so that the above mentioned method has problems in carrying developer to a development area uniformly when such a developer is used.
Furthermore, in most recent copying machines, a photo conductor and a development apparatus are combined and they can be easily exchanged, providing for labor savings related to maintenance by a service person. In such a system, since cost is higher if an exchanging cycle is short, a developer having a long service life and a latent image supporter having a service life comparable to the one of the developer, which is referred to below as a photo conductor, are required. However, when the two-component contact development method is employed, a photo conductor is always rubbed with developer, and, thus, it is easy for the photo conductor to become worn and difficult to have a long servicelife. Furthermore, density of a developer on a development area is required to be high to meet the demand for high image quality. However, if density of a developer on a development area is high, wear of the photo conductor is accelerated. In order to prevent wear of the photo conductor, prevention of wear by decreasing printing resistance has been attempted by adding a filler to the outermost layer of a photo conductor. Ozone generating from a charger and low resistance materials secondarily produced from nitrogen oxides fall on an outermost layer of a photo conductor and adhere to a surface of the layer. When a photo conductor in which a filler is not added to the outermost layer is used, abrasion of the outermost layer of a photo conductor is reduced and resistance of a surface of a photo conductor is reduced by adhesion of the low resistant materials, so that abnormal images having decrease in resolution or blur are not formed. The problem originates when wearing rate of the outermost layer is faster than the deposition rate of the low resistance materials. However, the abrasion loss of a photo conductor defines the service life of the photo conductor. On the other hand, when a layer having high wear resistance is laid on the outermost layer of a photo conductor as in the subject application, abrasion loss decreases and the service life of a photo conductor is not controlled by the wear resistance. However the deposition rate of the low resistance materials produced from ozone and nitrogen oxides, etc., described above overcomes the rate of wear resulting in deposition (adhesion) of the low resistance materials to the surface of the photo conductor. Gonsequently, side effects such as decrease in resolution and blur in an image resulting from decrease of resistance on a surface of a photo conductor are generated, therefore a new problem of the side effects controlling the service life of an image formation apparatus occurs.
When a layer including a filler is laid on the outermost layer of the above mentioned photo conductor, the wear resistance is improved, but side effects may occur. When a conductive filler is employed as a filler, resistance on a surface of a photo conductor is reduced, and decrease in resolution and blur in an image may occur due to a reason other than the above mentioned phenomenon. Especially, the phenomenon is significant when a photosensitive layer is made from an organic material. Therefore, it is necessary to employ a high resistance filler in an organic photo conductor. In this case, since the filler does not have charge transfer efficiency, when the photo conductor is repeatedly used in an electro-photographic apparatus, residual potential is elevated or electric potential of exposed areas is elevated in negative or positive development, so that there is produced a defect of decrease in image density.
Thus, as wear resistance of a photo conductor is improved and abrasion loss does not define the service life of a photo conductor, an electrostatic service life of the photo conductor defines the service life of the photo conductor. Specifically, point defects (stains and black points, etc.) on image background (white background), which are not in an original image, occur due to decreasing electrostatic property of a photo conductor (especially, local leak of electric potential). The defects may be taken for points in a drawing or period and comma, etc., in a draft in English so that the defects are crucial in an image.